Conventional pulse radar, fill level measuring devices have regularly a transmission system having a pulse producing system connected to a control unit. The pulse producing system produces for each measurement a transmission signal, which is composed of microwave pulses of a fixedly predetermined center frequency and a predetermined pulse repetition rate. The microwave pulses have, for example, fixedly predetermined center frequencies of 26 GHz or 78 GHz. The antenna is mounted on the container above the highest fill level to be measured, oriented toward the fill substance and sends the transmission signals into the container. Subsequently, the antenna receives as received signals the signal fractions reflected on the fill substance back toward the fill-level measuring device after a travel time dependent on the distance to the fill substance. The received signals are fed to a signal processing system connected to the transmission system and to the antenna, and the signal processing system determines the fill level based on the received signals.
In such case, measurement curves are regularly derived, which show the amplitudes of the received signals as a function of their travel time required for the path to the fill substance and back. From the travel times of the peaks of these measurement curves, then, based on the propagation velocity of the microwave pulses, the distance of the fill substance from the fill-level measuring device can be determined.
For fill level measurement today, a large number of different evaluation methods, frequently referred to as echo recognition methods, are applied, with which the measurement curves are used to ascertain which peak is to be attributed to the reflection on the surface of the fill substance. For example, the first occurring peak or the peak having the greatest amplitude can be selected as the peak of the particular measurement curve to be attributed to the reflection on the surface of the fill substance. From the travel time associated with this peak, based on the propagation velocity of the microwave pulses, the distance of the surface of the fill substance from the fill-level measuring device is derived, which then, based on the installed height of the antenna, is convertible into the fill level—thus the fill level of the fill substance in the container.
These fill level measuring devices deliver reliable measurement results in a large number of different applications. For fill level measurement of bulk goods, however, such measuring devices are, as a rule, not optimally suitable, since bulk goods regularly form hill and valley shaped bulk goods cones, whose surface profile is not registered with these fill-level measuring devices, so that a surface profile cannot be taken into consideration for the fill level determination.
Likewise, in given cases, problematic is the use of conventional fill level measuring devices with a single, rigidly mounted antenna in applications, in which objects (hereinafter referred to as disturbances) installed in the container protrude laterally into the beam path of the transmission signals. Thus objects, such as other measuring devices or filling nozzles, are present.
DE 102012109101 A1 describes a fill-level measuring device for measuring a fill level of a fill substance in a container. Such fill-level measuring device includes an antenna, which sends transmission signals with different center frequencies into the container, and receives as received signals their signal fractions reflected back in the container in the direction of the antenna. Furthermore, the antenna has different spatial radiation characteristics for different center frequencies depending on the center frequencies of the transmission signals. A signal processing system evaluates the received signals based on the center frequencies of the microwave pulses of the associated transmission signals and the center frequency dependence of the spatial radiation characteristics of the antenna.
Disadvantageous in the case of such a fill-level measuring device is that the center frequencies of the transmission signals must be changed, in order to change the radiation characteristics of the microwave pulses of the transmission signals. That means that the transmission signals of such fill-level measuring devices are broadband signals.
Fill level measuring devices with broadband transmission signals are not able to resolve the separation between the antenna and the fill substance finely and therefore are not able to determine the fill level as exactly as might be desired.